1. Field of the Invention
The present invention relates generally to pressure to electrical signal transducers or transmitters, and relates specifically to that type of such transmitters which employs a differential pressure sensor or sensing element to produce an electrical signal of a value dependent upon the difference between two fluid pressures. Transmitters of this type are referred to herein as differential pressure transmitters. More specificially, the invention relates to transmitters of this type which employ barrier diaphragms and fill fluids for transmitting fluid pressures to the sensor.
2. Description of the Prior Art
A typical transmitter of the known form last referred to above includes so-called pressure sensing head structure which contains two barrier diaphragms. A chamber on one side of one diaphragm is pressurized with a fluid having a first pressure, and a chamber on one side of the other diaphragm is pressurized with a fluid having a second pressure. These two fluids, hereinafter referred to as applied fluids, may well be so-called process fluids which are on the two sides of an orifice plate in a process flow line.
A separate chamber on the other side of each of the diaphragms contains an incompressible fill fluid. These fill fluids communicate with the opposite sides of a differential pressure sensor or sensing element, and the latter is displaced or deflected by an amount which is dependent upon the difference between the two pressures of the fill fluids, and which is thus dependent upon the differential pressure of the two applied fluids. The sensor produces an electrical output signal which is dependent upon said displacement and hence upon said differential pressure.
In early forms of such transmitters, each of the barrier diaphragms was essentially a flat membrane or plate. The pressure of the applied fluid on one side of the diaphragm caused it to deflect from its flat condition and hence to transmit the applied fluid pressure to the fill fluid on the other side of the diaphragm. The fill fluid, in turn, transmitted this pressure to the corresponding side of the sensor.
Although such flat barrier diaphragms were desirable because of their simplicity, it was found that they introduced unacceptable errors into the pressure measurements made by the transmitters employing them. Specifically, it was found that the pressure vs. deflection characteristic of such a flat diaphragm suffered from discontinuities, irregularities, and hysteresis as a result of the deflection of the diaphragm about its flat condition. This irregular or inconsistent behavior of the flat diaphragm, known as "oil canning", made it undesirable for use as a barrier diaphragm.
The foregoing problem was largely solved by replacing the flat diaphragm with one having circular convolutions or corrugations. However, such a diaphragm and its use have certain shortcomings. In the first place, special fabricating procedures are required to provide the proper convoluted or other non-flat diaphragm configuration.
Secondly, such a diaphragm configuration does not lend itself to those constructions wherein overpressure protection is to be provided by the use of barrier diaphragm back-up surfaces. As used herein, the term overpressure identifies any value of the differential pressure of the applied fluids which is out of the range of differential pressures that the transmitter is designed to handle and which would produce an unsafe pressure difference across the sensor. In such a construction, each barrier diaphragm is provided with a back-up surface, on which that diaphragm is to bottom-out if the pressure of the applied fluid acting on that diaphragm becomes sufficiently higher than the pressure of the other applied fluid to cause the difference between these pressure to closely approach an overpressure value. Such a bottomed-out diaphragm is protected from being damaged by the pressures producing the overpressure condition, and ideally also prevents these pressures from producing an unsafe pressure difference across the sensor.
For the above-described protection to be effective, the mating surfaces of a barrier diaphragm and its back-up member must match to such a degree that the applied pressure does not further deflect the bottomed-out diaphragm sufficiently to cause a damaging differential pressure to be applied to the sensor. In numerous constructions, it has been found to be difficult, if not impossible, to match the mating surfaces of a convoluted or other non-flat diaphragm and its back-up member to the degree required to fully protect the sensor. Stated differently, it has been found that inherent irregularities in the match between the mating convoluted barrier diaphragm and back-up surfaces allow the above-noted further, damaging deflection of the diaphragm to occur, and thus prevent the back-up surface from being sufficient to properly protect the sensor in certain constructions.
The foregoing is particularly true of those constructions wherein the sensor is of the minute deflection type. Such a sensor has a practically negligible volume displacement, and is readily degraded or damaged by an excession differential pressure. A semiconductor strain gauge element in an example of such a sensor which requires the above-noted degree of surface match which is not readily obtainable in practice.
Because of the shortcoming or deficiency of the convoluted barrier diaphragm as just described, it has been found to be inadvisible to rely on the use of the back-up member method for obtaining sensor overpressure protection in transmitters which employ convoluted or other non-flat barrier diaphragms. This has required that such transmitters be provided with supplemental overpressure protecting means, with the result that such transmitters are unduly large, complex, and/or costly. An example of a transmitter requiring and embodying such suppplement overpressure protecting means is found in the U.S. Pat. 3,841,158.